1. Field of the Invention
The present invention relates to a twin phase locked loop control double super tuner circuit which is used to receive analog broadcasts and a digital broadcasts of a CATV (Cable Television) broadcasting system, satellite broadcasting system, and HDTV (High Definition Television) system using the UHF (Ultra High Frequency) band or the like.
2. Description of the Related Art
It is well known that a tuner which is used to receive analog broadcasts and digital broadcasts of a CATV broadcasting system, satellite broadcasting and HDTV broadcasting in system using the UHF band or the like, is usually called a double super tuner. Such tuner comprises two frequency converters and two local oscillators.
FIG. 6 shows the conventional double super tuner. In such a double super tuner, a high frequency signal S1 is applied to its input terminal 11. This high frequency signal S1 first passes through a wide-band Band Pass Filter 12 and is amplified by a Radio Frequency amplifier 13. The resulting high frequency signal S2 is fed to one input terminal of a first frequency converter 14. A first local oscillation signal S3 generated by a first local oscillator 15 is supplied to its other input terminal. The first frequency converter 14 mixes the high frequency signal S2 with the first local oscillation signal S3 to obtain a first intermediate frequency signal S4. That is, the first frequency converter 14 up-converts the high frequency signal S2 to a predetermined first intermediate frequency.
A balanced mixer is typically used as the first frequency converter 14. The first intermediate frequency signal S4 from the first frequency converter 14 is filtered by BPF 16 of the intermediate frequency band. The resulting first intermediate frequency S5 is amplified by an amplifier 17 and passes through a BPF 18. The BPF 18 is the same as the BPF 16 in this configuration. The first intermediate frequency signal S6 from the BPF 18 is fed to one input terminal of a second frequency converter 19. A second local oscillation signal S7 is supplied to the other input terminal of the second frequency converter 19 from a second local oscillator 20. The second frequency converter 19 mixes the first intermediate frequency signal S6 with the second local oscillation signal S7 to obtain a second intermediate frequency signal S8.
The second frequency converter 19 down-converts the first intermediate frequency signal S6 to a predetermined second intermediate frequency. The second intermediate frequency signal S8 from the second frequency converter 19 passes through a BPF 21 of the second intermediate frequency band and is amplified by an amplifier 22. The resulting signal is output from an output terminal 23.
The first local oscillator 15 stated above is controlled by a PLL circuit 24. That is, the first local oscillation signal S3 from the first local oscillator 15 is fed not only to the first frequency converter 14, but also to the PLL circuit 24. The PLL circuit 24 compares the phase of the oscillation signal by dividing the signal by n, then comparing the resulting signal with a signal resulting from the division of a reference oscillation signal, corresponding to a preset reference value, by m. The PLL circuit 24 generates a phase error signal S9 corresponding to the comparison result. The phase error signal S9 is converted into a tuning voltage level signal V1 by an active (Low Pass Filter(LPF) 25 and sent to the first local oscillator 15 in the form of a control voltage. Therefore, the first local oscillator 15 is so controlled that the oscillation frequency of the first local oscillation signal S3 becomes a frequency corresponding to the tuning voltage level signal V1 (that is, a frequency obtained by multiplying the reference oscillation signal frequency by n/m).
The second local oscillator 20 is controlled by an AFC (Automatic Frequency Control) voltage which is usually supplied to a terminal 26 from a video demodulating part (not shown).
In recent years, with the adoption of wide band for the CATV broadcasting system, hither frequencies have been used as the first intermediate frequency signal S4 to expand the receiving frequency range of the double super tuner. As a result, higher frequencies have been adopted for the local oscillators 15, 20, so the frequency fluctuation range with respect to a control voltage has become wider. Therefore, it has become difficult to control such frequency fluctuations. For this reason, instead of the conventional AFC for the second local oscillator 20, a twin PLL control double super tuner system which PLL-controls both the first local oscillator 15 and the second local oscillator 20 has been increasingly adopted. That is, the twin PLL control double super tuner system includes two frequency converters, two local oscillators and two PLL circuits.
To cope with digital TV broadcasting, the development of a low phase noise tuner (that is, a tuner with good phase noise characteristic) is in demand, one which can receive digitally modulated TV signals such as 16QAM (Quadrature Amplitude Modulation), 64QAM and 16VSB (Vestigial Side Band). Therefore, use of the twin PLL control double super tuner system is increasing. Especially, when as a CATV transmitter sends out both analog broadcasts wave and the digital broadcasts, so a CATV receiver tuner must be able to receive both broadcasts.
However, for the tuner of the conventional twin PLL control double super tuner type, high PLL phase comparison frequency is required to produce a tuner of low phase noise which can receive the digital broadcasts. In that case, the AFC does not work well at the reception of the analog broadcasts.
In the case of the tuner circuit of the conventional twin PLL control double super tuner type, if a reference oscillation injection line (pattern) is installed from a first PLL circuit to a second PLL circuit in the tuner in order to share a PLL reference oscillation signal from the first local oscillator 15 and the second local oscillator 20, the line is electomagnetically coupled with the surrounding circuits. As a result, the tuner is apt to produce adverse effects such as an electromagnetic wave radiation and noise intrusion from the environment.
As described above, where the tuner circuit of the conventional twin PLL control double super tuner type receives both the digital broadcasts wave and the analog broadcasts, a high PLL phase comparison frequency is required to obtain a tuner of good phase noise characteristic for the reception of the digital broadcast wave. In this case, the AFC does not work well for the reception of the analog broadcast wave. Installation of a reference oscillation signal injection line in the tuner to share the PLL reference oscillation signal from the first local oscillator and the second local oscillator causes electromagnetic coupling with the surrounding circuits. As results, adverse effects such as an electric wave radiation and a noise intrusion result.